1. Field of the Invention
The present invention relates to digital electronic systems, and, in particular, to circuitry used in digital computers and the like. More particularly, the invention relates to the suppression of radiated electrical noise generated by the signals of such systems.
2. Description of the Related Art
The circuitry of digital electronic systems is typically mounted on printed circuit boards. For example, in small business or personal computers, such circuit boards include a main system board (often referred to as a mother board) and a number of smaller circuit boards that are inserted into connectors on the mother board. Typically, such circuit boards are multi-layer circuit boards. That is, the connection to and the interconnections between integrated circuits and other components on the surface of the circuit board are provided by metallic traces on the surface layers of the circuit board as well as on additional traces on inner layers or the bottom of the circuit board. In addition, exemplary multi-layered printed circuit boards include layers that are dedicated to a particular voltage reference. For example, the ground reference (i.e., 0 volts), may be distributed on an inner layer of the circuit board. In addition, the primary supply voltage (e.g., +5 volts) may also be distributed on a separate dedicated layer of the circuit board. Interconnections between the layers of the circuit board are provided by feed-through connections (often referred to as vias). In particular, connections between integrated circuits on the surface layers and circuit paths on the opposing surface layer are provided by vias. Similarly, the power and ground connections to the integrated circuits are provided by vias that are electrically connected to the respective voltage reference layers.
In many instances, digital electronic systems, and particularly digital computer systems, include the generation of relatively high frequency analog signals. For example, a typical microcomputer, such as an IBM PC compatible computer, generates the video signal for driving an associated video display. Furthermore, such digital electronic systems include the generation of high frequency digital signals. For example, a typical system based upon an Intel.RTM. 80386 microprocessor operating at 25 MHz utilizes a system clock having a frequency of 50 MHz.
The high frequency signals generated by the digital systems are typically carried by the metal traces of the circuit board described above. The signals on the traces have the potential to radiate out of the system so as to interfere with the operation of television receivers and other radio frequency communications equipment. Such radio frequency interference (RFI) is not permitted, according to regulations promulgated by the Federal Communications Commission (FCC). Thus, it is necessary to suppress the radiation of electromagnetic emissions in order to comply with FCC regulations.
Additionally, the signals generated by the digital systems must in many cases be interfaced to external equipment, such as the video display, networking equipment, and the like. Therefore, the traces of the circuit board which carry the required signals must at some point interface with connectors and/or cables leading to the external devices. However, the impedance of the circuit board traces and the cables are typically drastically different. This mismatch in impedance leads to substantial undesired signal reflections which are in turn the source of further RFI.
One method of suppressing the radiation of RFI from the traces of a circuit board is to route the traces in a twisted-pair pattern. The twisted-pair carries a differential signal. The signal is represented by the difference between the voltage level of a "signal" line and the voltage level of a "return" line. The signal and the return lines of the twisted-pair traces criss-cross each other so as to form a sequence of small "current loops." Each current loop circulates current in the opposite direction. Each of the loops behaves as a miniature loop antenna, with current "circulating" just as though through a loop antenna. Since the current circulation is in opposite direction in adjacent loops, opposite electromagnetic fields are generated. The net field observed from far enough away (so that the various loops appear collectively as the same point source) tends to zero.
One failing of the twisted-pair scheme is that there must always be two "pigtail" areas, where the twisted-pair trace connects to the signal source or the load. The pigtail areas do not form opposing current loops. Rather, the pigtail portions merely comprise two separate differential conductors, side by side. The difference in voltage of the two conductors gives rise to electromagnetic emissions which are not cancelled out. Consequently, the pigtails are the source of undesired RFI.
Another failing of the twisted-pair scheme is that the fields effectively cancel out only at a distance where the length of the twisted-pair trace appears as a point. At close distances, the differing fields from each separate current loop of the length of twisted-pair can be detected. Consequently, undesired RFI will be seen at short distances, e.g., by sensitive components on the same board as the twisted-pair traces.
A further disadvantage of twisted-pair traces on circuit boards is the typical severe mismatch of impedance between the traces of the circuit board and the external cables to which they connect. The impedance mismatch leads to signal reflections which are the source of undesired RFI.
A printed circuit in accordance with the present invention overcomes these and other disadvantages and provides improved suppression of electromagnetic emissions.